A peristaltic pump displaces liquid by repetitively compressing a section of the flexible tube comprising an IV line. This line is threaded through a channel formed in the pump and extends unbroken, from a drug container to a patient. One type of peristaltic pump has a linear channel in which the IV line is threaded and is thus referred to as a linear (or traveling wave) peristaltic pump. The linear peristaltic pump includes a plurality of finger-like plungers that are sequentially actuated by cams mounted along a motor driven shaft. Liquid within the section of tubing that extends along the linear channel is advanced along the tubing's longitudinal axis by the advancing wave-like compression of the fingers. An example of such a pump is disclosed in U.S. Pat. No. 4,479,797.
Inlet and outlet valves and a single liquid displacement plunger are used in another type of peristaltic pump. Each pumping cycle in this type of pump begins with the outlet valve closed and the inlet valve open. Fluid flows from the source container into a short section of tubing that is disposed between the inlet and outlet valve. After this section of tubing has filled with liquid, the inlet valve closes and the outlet valve opens. The plunger then compresses the short section of tubing between the valves, displacing the liquid contained therein, and forcing it from the pump. U.S. Pat. No. 4,559,038 discloses a peristaltic pump of this type.
The rate at which fluid is delivered by each type of positive displacement pump discussed above is normally controlled by the rate at which the pump operates, e.g., the rotational rate of the rotating arm in that type of peristaltic pump. Furthermore, the accuracy with which a given rate or volume of fluid flow can be achieved by these pumps is dependent upon the pressure of the fluid at the input of the pump and the back pressure at its output. Since the flexible tubing in a peristaltic pump defines a compliant pumping chamber, the volume of fluid that fills the pumping chamber is affected by the head pressure of the fluid from the drug container. Similarly, the volume of fluid delivered at the output of the pump depends on the back pressure of the fluid downstream of the outlet. The single plunger type of peristaltic pump has positive closure inlet and outlet valves and is particularly sensitive to head and back pressures because the volume of the pumping chamber disposed between the valves and the amount of fluid that fills the chamber generally must be constant to provide an accurate and consistent rate of flow from the pump. A superior single plunger peristaltic pump that is capable of supplying fluid at a desired flow rate and pressure, generally independent of head pressure and back pressure downstream of the pump is disclosed in the above referenced commonly assigned U.S. patent application Ser. No. 494,210, filed Mar. 15, 1990.
In the single plunger type of peristaltic pump, the plunger should compress the tubing uniformly and consistently with each pumping stroke to provide an accurate and consistent rate of fluid flow from the pump. Since the plunger is typically driven by a rotating cam, it is essential that the cam rotate smoothly at a constant speed. The plunger and valves in the peristaltic pump are actuated by cam followers that ride along radially varying profiles defining different circumferential tracks on the cam. As the cam followers roll along the tracks, the slope of each track's profile causes a tangential force to be developed that produces a rotational torque tending either to aid or to impede the rotation of the cam. The net torque thus developed from the forces exerted by all of the cam followers can cause the rotational speed of the cam to increase or decrease at various points in its rotation, due to changes in the load presented to a motor that rotatably drives the cam. Such rotational speed changes affect the accuracy with which the pump delivers fluid, assuming that servo-speed control is not provided, and adversely impacts audible noise and power consumption.
In consideration of these problems that exist with the cam assemblies in prior art peristaltic pumps and other devices using cam drives, it is an object of the present invention to provide a torque compensated cam assembly that maintains a substantially constant load on a prime mover used to rotate a cam in the cam assembly. Another object of this invention is to provide a cam assembly in which the cam rotates at a rate substantially independently of loading developed due to interaction between one or more cam followers and tracks formed on the cam. Yet a further object is to provide a cam assembly that can be used in a positive displacement fluid pump to deliver fluid to an output port of the pump at a predefined pressure. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follows.